61A Lecture 27 Wednesday, October 31 Programming Languages - PowerPoint PPT Presentation

61A Lecture 27 Wednesday, October 31

Programming Languages Computers have software written in many different languages. Machine languages: statements can be interpreted by hardware • All data are represented as sequences of bits • All statements are primitive instructions High-level languages: hide concerns about those details • Primitive data types beyond just bits • Statements/expressions can be non-primitive (e.g., calls) • Evaluation process is defined in software, not hardware High-level languages are built on top of low-level languages Machine C Python language 2

Metalinguistic Abstraction Metalinguistic abstraction : Establishing new technical languages (such as programming languages) f ( x ) = x 2 − 2 x + 1 λ f. ( λ x.f ( x x ))( λ x.f ( x x )) In computer science, languages can be implemented : • An interpreter for a programming language is a function that, when applied to an expression of the language, performs the actions required to evaluate that expression. • The semantics and syntax of a language must be specified precisely in order to build an interpreter. 3

The Scheme-Syntax Calculator Language A subset of Scheme that includes: • Number primitives • Built-in arithmetic operators: +, -, *, / • Call expressions > (+ (* 3 5) (- 10 6)) 19 > (+ (* 3 (+ (* 2 4) (+ 3 5))) (+ (- 10 7) 6)) 57 4

Syntax and Semantics of Calculator Expression types : • A call expression is a Scheme list • A primitive expression is an operator symbol or number Operators : • The + operator returns the sum of its arguments • The - operator returns either  the additive inverse of a single argument, or  the sum of subsequent arguments subtracted from the first • The * operator returns the product of its arguments • The / operator returns the real-valued quotient of a dividend and divisor (i.e., a numerator and denominator) 5

Expression Trees A basic interpreter has two parts: a parser and an evaluator scheme_reader.py scalc.py lines Parser expression Evaluator value '(+ 2 2)' Pair('+', Pair(2, Pair(2, nil))) 4 '(* (+ 1' Pair('*', Pair(Pair('+', ...))) 4 ' (- 23)' printed as ' (* 4 5.6))' (* (+ 1 (- 23) (* 4 5.6)) 10) ' 10)' Lines forming A number or a Pair with an A number a Scheme operator as its first element expression 6

Syntactic Analysis Syntactic analysis identifies the hierarchical structure of an expression, which may be nested. Each call to scheme_read consumes the input tokens for exactly one expression. '(', '+', 1, '(', '-', 23, ')', '(', '*', 4, 5.6, ')', ')' Base case: symbols and numbers Recursive call: scheme_read sub-expressions and combine them Demo ( http://inst.eecs.berkeley.edu/~cs61a/fa12/projects/scalc/scheme_reader.py.html ) 7

Evaluation Evaluation discovers the form of an expression and then executes a corresponding evaluation rule. • Primitive expressions are evaluated directly. • Call expressions are evaluated recursively:  Evaluate each operand expression  Collect their values as a list of arguments  Apply the named operator to the argument list Demo 8

Applying Operators Calculator has a fixed set of operators that we can enumerate def calc_apply(operator, args): """Apply the named operator to a list of args.""" if operator == '+': Dispatch on operator name return ... if operator == '-': ... ... Demo 9

Read-Eval-Print Loop The user interface to many programming languages is an interactive loop, which • Reads an expression from the user, • Parses the input to build an expression tree, • Evaluates the expression tree, • Prints the resulting value of the expression. Demo 10

Raising Application Errors The sub and div operators have restrictions on argument number. Raising exceptions in apply can identify such issues: def calc_apply(operator, args): """Apply the named operator to a list of args.""" ... if operator == '-': if len(args) == 0: raise TypeError(operator + ' requires at least 1 argument') ... ... if operator == '/': if len(args) != 2: raise TypeError(operator + ' requires exactly 2 arguments') ... 11

Handling Errors The REPL handles errors by printing informative messages for the user, rather than crashing. Demo A well-designed REPL should not crash on any input! 12